This invention relates to a manipulator and its control method, and more particularly to a manipulator and its control method used for manipulators having force sensors, operating in space.
As disclosed in "The Space Manipulator System Controlled with Terminal Motion Corresponding Control Scheme" presented at the Fourth Symposium on Space Station (Apr. 28, 1988), the prior-art control method of the manipulator is by use of a force sensor mounted to the tip of the slave arm having six degrees of freedom of movement in a manipulator system comprising a master arm and a slave arm, to detect a load applied to the manipulator and to feed back the detected load to the master arm, thereby conveying the feeling about the magnitude of the force to the operator.
The above-mentioned prior art system has a problem of low work efficiency because when an overload is detected by the force sensor, the operation of the manipulator has to be brought to a complete stop. Since no means is available for lessening a detected overload, which is applied to the structural members, such as the arm and the joint mechanism of the manipulator, the structural members have had to be so constructed as to have a sufficient safety factor of strength in order to secure a required service life. This has been an obstacle to achieving reductions in size and weight of the manipulator. The speed reducers provided at joints of the manipulator have a function to decelerate the output from the motor to produce a torque required to drive the joints. When the speed reducer is accelerated from the output side, it is necessary to limit the load to a smaller amount than that during its deceleration. If the manipulator is formed with a sufficient safety factor of strength, this places constraints on the attempts to make smaller and lighter joints for use with the manipulator.